OpenAI Chat Completions

通过兼容端点接入现有 OpenAI 客户端。

POST /v1/ai/chat/completions 是 Downcity Model Protocol 外层的兼容 Adapter：

OpenAI request
  -> ModelCall
  -> AIService / AIChannel
  -> ModelStreamEvent
  -> OpenAI JSON 或 SSE

curl "$FEDERATION_URL/v1/ai/chat/completions" \
  -H "Authorization: Bearer $USER_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "deepseek-v4-flash",
    "messages": [{"role":"user","content":"你好"}],
    "stream": true
  }'

Messages、tools、tool choice、图片、reasoning、usage 和 finish reason 都会转换到同一条 Downcity 执行链。该端点不是 raw passthrough。